The field of this invention is that of tools used for the cleaning of pipelines, especially the long extended reach pipelines in offshore areas. As gases are produced in deep offshore wells and flow along pipelines, they may combine with water within the gas stream at certain combinations of temperature and pressure to form hydrate blockages. The hydrates are similar to crushed ice packed within the pipeline.
Once formed, the hydrate blockages can take weeks or months to disassociate or melt, with the production stream thru the pipeline being completely blocked. The most common means to remove the blockage is to reduce the pressure on the accessible side of the blockage to a lower pressure which allows it to melt. When this happens, the higher pressure gas on the other side of the blockage will tend to expand as it moves through the porosity of the blockage. This gas expansion provides a cooling which prolongs the life of the hydrate. This process can take weeks or months.
A second method for removing the hydrate blockage is to place a coiled tubing within the pipeline to circulate liquids to dissolve the hydrate directly. In shorter pipelines the coiled tubing can be simply pushed out into the pipeline. In longer ones, a rubber cup “pig” is placed on the end of the pipeline to pull the coiled tubing out to the location.
A potential problem can occur with this method. If the liquids are circulated out to the blockage through the annular area between the outside diameter of the coiled tubing and the inside diameter of the pipeline and then return within the internal diameter of the coiled tubing, the internal diameter of the coiled tubing can become blocked. The hydrates may reform within the bore of the coiled tubing and make the coiled tubing and associated pig difficult to retrieve from the pipeline, much less removing the pipeline blockage.
A similar situation may occur if paraffins are formed within the internal diameter of the pipeline and are removed through the internal bore of the coiled tubing.